82 MOLLUSCA. 



of the branchiae previously to being brought to light *. All the Ace- 

 phala are aquatic f. 



ORDER I. 



ACEPHALA TESTACEA. 



Testaceous Acephala, or Acephala with four branchial leaflets J, 

 are beyond all comparison the most numerous. All the bivalves, and 

 some genera of the multivalves belong to this order. Their body, 

 which contains the liver and viscera, is placed between the two lami- 

 nae of the mantle ; forwards, and still between these laminae are the 

 four branchial leaflets, transversely and regularly striated by the ves- 

 sels : the mouth is at one extremity, the anus at the other, and the 

 heart towards the back ; the foot, when it exists, is inserted between 

 the four branchiae. On the sides of the mouth are four triangular 

 leaflets, which are the extremities of the two lips, and serve as tenta- 

 cula. The foot is a mere fleshy mass, the motions of which are 

 effected by a mechanism analogous to that which acts on the tongue 

 of the Mammalia. Its muscles are attached to the bottom of the valves 

 of the shell. Other muscles, which sometimes form one mass and 

 sometimes two, cross transversely from one valve to the other to keep 

 them closed, but when the animal relaxes these muscles, an elastic 

 ligament placed behind the hinge opens the valves by its contraction. 



A considerable number of bivalves are provided with what is termed 

 a byssus, or a fasciculus of threads more or less loosely connected, 

 which issues from the base of the foot, and by which the animal ad- 

 heres to various bodies. It uses its foot to direct the threads and to 

 agglutinate their extremities ; it even reproduces them when cut, but 

 the nature of the production is not thoroughly ascertained. Reaumur 

 considered these threads as a secretion, spun and drawn from the 

 sulcus of the foot ; Poli thinks they are mere prolongations of tendi- 

 nous fibres. 



* Some naturalists are of the opinion that the very minute bivalves, which in cer- 

 tain seasons fill the external branchiae of the Anodontes and Mytilus, are not the progeny 

 of those Mollusca, but a different and parasitic species. See, on this subject, the 

 Dissertation of M. Jacobseu. The difficulty seems to be removed by the observations 

 of Sir Ev. Home. 



f M. de Lamarck at first changed my name of Acephala into that of Acephalata. 

 M. de Blainville forms a class, which he calls ACEPHALOPHORA, from my Acephala 

 and my Bruchioj-ola. 



+ M. de Lamarck, in his last work, has made his class of the CONCHIFERA from 

 my Testaceous Acephala; and M. de Blainville has converted the same into his order 

 of the ACEPHALOPHORA LAMELLIBRANCHIATA : but it is always the same thing. 



